Syd teen asked for a lighter, then stabbed

The man who critically injured a teenage boy in an unprovoked stabbing outside a Sydney shopping centre asked his victim for a cigarette lighter immediately before the attack.

The 16-year-old local boy had been buying groceries at Woolworths in Balmain with his brother and a teenage friend on Thursday evening when they were confronted by a man in the supermarket's rear car park.

The man asked the teens for a lighter in a "one-way conversation", NSW Police said on Friday.

"What followed was a swift and unprovoked attack, being a stabbing to the abdomen area of the male and stab wounds to his hand," Acting Superintendent Anthony Bell told reporters in Sydney.

The attacker fled on foot, along with a female associate, before paramedics arrived.

The boy was taken to Royal Prince Alfred Hospital, where he underwent emergency surgery. He had another operation on Friday afternoon.

Riot police and forensic dogs spent Thursday night and Friday conducting extensive searches of the area but have been unable to find the attacker.